The major thrust of this study is to elucidate the basis of neoplastic transformation of normal tissues. Two experimental model systems were used in this project: (1) Intracisternal A particle (IAP) containing neoplasms, a BALB/c mammary tumor cell line, its hybrid and cybrid clones, a plasma cell tumor (MOPC 104E), and feral mouse cell lines of both Mus musculus and Mus cervicolor; and (2) a rat hepatoma induced by the chemical carcinogen, N,N-2,7-fluorenylene bis-2,2,2-trifluoroacetamide. Multidisciplinary approaches were required in this investigation. They involve tissue culture, biochemical and biophysical analyses, cytogenetics, immunology and tumor biology. Since we have completed extensive characterizations morphologically, biologically, biochemically and immunologically of both these retroviruses, our current emphasis of research efforts focuses on: (1) integrations of horizontally transferred viral genome and its localization in the host cell; (2) regulation of viral gene expressions; (3) integration site(s) for exogenous viral DNA sequences; (4) viral gene conservation during the evolution of the Mus genus; and (5) resolution of the GAG and ENV gene products (gs antigens and envelope antigen).